Pharmacokinetic equations for several theoretical models were derived: 1) Based on a single compartment for a drug and one of its primary metabolites several plotting techniques were invented for estimating the rate constant of elimination of the metabolite in the presence of the parent drug. 2) Plotting techniques were invented to evaluated the relative importance of decomposition pathways of chemically reactive metabolites. 3) Equations were derived for the area under the curve of a general model for drug disposition. 4) The meaning of the relative areas under the curves of drugs and their metabolites has been clarified.